Shocks in the Warm Neutral Medium I -- Theoretical model

Benjamin Godard, Guillaume Pineau Des Forêts, Shmuel Bialy

Published 2024-04-30Version 1

Context. Atomic and molecular line emissions from shocks may provide valuable information on the injection of mechanical energy in the interstellar medium (ISM), the generation of turbulence, and the processes of phase transition between the Warm Neutral Medium (WNM) and the Cold Neutral Medium (CNM).Aims. In this series of papers, we investigate the properties of shocks propagating in the WNM. Our objective is to identify the tracers of these shocks, use them to interpret ancillary observations of the local diffuse matter, and provide predictions for future observations. Methods. Shocks propagating in the WNM are studied using the Paris-Durham shock code, a multi-fluid model built to follow the thermodynamical and chemical structures of shock waves, at steady-state, in a plane-parallel geometry. The code, already designed to take into account the impact of an external radiation field, is updated to treat self-irradiated shocks at intermediate ($30<V_S <100$ km s$^{-1}$) and high velocity ($V_S \ge 100$ km s$^{-1}$) which emit ultraviolet (UV), extreme-ultraviolet (EUV), and X-ray photons. The couplings between the photons generated by the shock, the radiative precursor, and the shock structure are computed self-consistently using an exact radiative transfer algorithm for line emission. The resulting code is explored over a wide range of parameters ($0.1 \le n_H \le 2$ cm$^{-3}$, $10 \le V_S \le 500$ km s$^{-1}$, and $0.1 \le B \le 10$ $\mu$G) that covers the typical conditions of the WNM in the solar neighborhood. Results. The explored physical conditions are prompt to the existence of a diversity of stationary magnetohydrodynamic solutions, including J-type, CJ-type, and C-type shocks. These shocks are found to naturally induce phase transition between the WNM and the CNM, provided that the postshock thermal pressure is larger than the maximum pressure of the WNM and that the maximum density allowed by magnetic compression is larger than the minimum density of the CNM. The input flux of mechanical energy is primarily reprocessed into line emissions from the X-ray to the submillimeter domain. Intermediate and high velocity shocks are found to generate a UV radiation field that scales as $V_S^3$ for $V_S < 100$ km s$^{-1}$ and as $V_S^2$ at higher velocities, and an X-ray radiation field that scales as $V_S^3$ for $V_S \ge 100$ km s$^{-1}$. Both radiation fields may extend over large distances in the preshock depending of the density of the surrounding medium and the hardness of the X-ray field which is solely driven by the shock velocity. Conclusions. This first paper presents the thermochemical trajectories of shocks in the WNM and their associated spectra. It corresponds to a new milestone in the development of the Paris-Durham shock code and a stepping stone for the analysis of observations that will be carried out in forthcoming works.